Polishing pad

ABSTRACT

A polishing pad capable of maintaining a high level of dimensional stability upon moisture absorption or water absorption and providing high polishing rate includes a polishing layer of a polyurethane foam having fine cells, wherein the polyurethane foam includes a cured product of a reaction of (1) an isocyanate-terminated prepolymer (A) that includes an isocyanate monomer, a high molecular weight polyol (a), and a low molecular weight polyol, (2) an isocyanate-terminated prepolymer (B) that includes a polymerized diisocyanate and a polyethylene glycol with a number average molecular weight of 200 to 1,000, and (3) a chain extender.

REFERENCE TO RELATED APPLICATIONS

This application is a national stage application under 35 USC 371 ofInternational Application No. PCT/JP2008/054583, filed Mar. 13, 2008,which claims the priority of Japanese Patent Application No.2007-084785, filed Mar. 28, 2007, the contents of both of which priorapplications are incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to a polishing pad capable of performingplanarization of materials requiring a high surface planarity such asoptical materials including a lens and a reflecting mirror, a siliconwafer, a glass substrate or an aluminum substrates for a hard disc and aproduct of general metal polishing with stability and a high polishingefficiency. A polishing pad of the invention is preferably employed,especially, in a planarization step of a silicon wafer or a device onwhich an oxide layer or a metal layer has been formed prior to furtherstacking an oxide layer or a metal layer thereon.

BACKGROUND OF THE INVENTION

Typical materials requiring surface flatness at high level include asingle-crystal silicon disk called a silicon wafer for producingsemiconductor integrated circuits (IC, LSI). The surface of the siliconwafer should be flattened highly accurately in a process of producingIC, LSI etc., in order to provide reliable semiconductor connections forvarious coatings used in manufacturing the circuits. In the step ofpolishing finish, a polishing pad is generally stuck on a rotatablesupporting disk called a platen, while a workpiece such as asemiconductor wafer is stuck on a polishing head. By movement of thetwo, a relative speed is generated between the platen and the polishinghead while polishing slurry having abrasive grains is continuouslysupplied to the polishing pad, to effect polishing processing.

As polishing characteristics of a polishing pad, it is requested that apolished object is excellent in planarity and in-plane uniformity and apolishing rate is large. A planarity and in-plane uniformity of apolished object can be improved to some extent with a polishing layerhigher in elastic modulus. A polishing rate can be bettered byincreasing a holding quantity of a slurry on a foam with cells therein.

Polishing pads including a polyurethane foam are proposed as polishingpads that meet the above properties (see Patent Documents 1 and 2). Sucha polyurethane foam is produced by a reaction of anisocyanate-terminated prepolymer with a chain extender (curing agent),in which in view of hydrolysis resistance, elastic properties, wearresistance, or the like, a polyether (a polytetramethylene glycol with anumber average molecular weight of 500 to 1,600) or a polycarbonate ispreferably used as a high molecular polyol component for the isocyanateprepolymer. However, when the above polishing layer absorbs moisture orwater, the cohesion of its hard segment can be reduced so that itsdimensional stability can be easily reduced. The polishing pad also hasa problem in which in serious cases, it is warped or heaved so that itspolishing properties such as planarization properties and in-planeuniformity may gradually change.

Patent Document 3 discloses that in order to improve the retainabilityslurry, a polymer composition for polishing pads should show a volumeswelling rate of 20% or less when it is immersed in water at 23° C. for72 hours. However, such a polymer composition for polishing pads uses athermoplastic polymer and thus can hardly form a polishing pad that canmaintain a high level of dimensional stability when it absorbs moistureor water.

Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No.2000-17252

Patent Document 2: Japanese Patent No. 3359629

Patent Document 3: JP-A No. 2001-47355

SUMMARY OF THE INVENTION

An object of the invention is to provide a polishing pad capable ofmaintaining a high level of dimensional stability upon moistureabsorption or water absorption and providing high polishing rate, and toprovide a method for manufacturing the same. Another object of theinvention is to provide a method of manufacturing a semiconductor devicewith the polishing pad.

As a result of investigations to solve the problems, the inventors havefound that the objects can be achieved with the polishing pad describedbelow, and have completed the invention.

Specifically, the invention is related to a polishing pad including apolishing layer including a polyurethane foam having fine cells, whereinthe polyurethane foam includes a cured product of a reaction of

(1) an isocyanate-terminated prepolymer (A) includes an isocyanatemonomer, a high molecular weight polyol (a), and a low molecular weightpolyol,(2) an isocyanate-terminated prepolymer (B) includes a polymerizeddiisocyanate and a polyethylene glycol with a number average molecularweight of 200 to 1,000, and(3) a chain extender.

In conventional polishing layers, the cohesion of the hard segment canbe easily reduced during absorption of moisture or water. It isconsidered that this is because conventional polishing layers are madeof polyurethane foams whose hard segment is formed only by physicalcrosslinks and that, therefore, such polishing layers can moresignificantly undergo dimensional change due to elongation, warpage, orthe like as they absorb more moisture or water.

The inventors have found that use of an isocyanate-terminated prepolymer(A) including an isocyanate monomer, a high molecular weight polyol (a),and a low molecular weight polyol in combination with anisocyanate-terminated prepolymer (B) includes a polymerized diisocyanateand a polyethylene glycol with a number average molecular weight of 200to 1,000 and partial introduction of chemical crosslinks into a polymerby a reaction of these materials with a chain extender (partiallyforming a three-dimensional crosslink structure) allow the production ofa polishing layer whose hard segment has increased cohesion duringabsorption of moisture or water and which can maintain a high level ofdimensional stability. The chemically crosslinked network can beexpanded using the two prepolymers. In addition, a polyethylene glycolwith a number average molecular weight of 200 to 1,000 is used as apolyol component of the prepolymer (B), so that the resultingpolyurethane has an improved affinity for water and forms a highlywater-absorbing polyurethane foam. As a result, the polishing pad hasimproved slurry-holding ability and provides higher polishing rate.

The high molecular weight polyol (a) is preferably a polyether polyolwith a number average molecular weight of 500 to 5,000. The isocyanatemonomer is preferably toluene diisocyanate and dicyclohexylmethanediisocyanate. The polymerized diisocyanate is preferably a polymerizedhexamethylene diisocyanate of isocyanurate type and/or biuret type. Whenthese materials are used, the polyurethane foam can be produced withgood handling ability so that the invention can be more effective.

The isocyanate-terminated prepolymer (B) is preferably added in anamount of 5 to 30 parts by weight, based on 100 parts by weight of theisocyanate-terminated prepolymer (A). If the amount of addition of theprepolymer (B) is less than 5 parts by weight, the ratio of chemicalcrosslinks in the polymer can be insufficient so that the cohesion ofthe hard segment can be insufficient during absorption of moisture orwater and that it may tend to be difficult to maintain the high leveldimensional stability of the polishing layer. Moreover, it may tend tobe difficult to form a highly water-absorbing polyurethane foam. On theother hand, if the amount is more than 30 parts by weight, the contentof the chemically crosslinked portion in the polymer may be so high thatthe polishing layer may have too high hardness, which may reduce thein-plane uniformity of the object being polished or reduce the wearresistance of the polyurethane foam, so that the life of the polishingpad may tend to be short. In this case, the surface of the object beingpolished is also more likely to be scratched.

The polyurethane foam preferably has an average cell diameter of 20 to70 μm and a cut rate of 2 μm/minute or less. If the average celldiameter deviates from the range, the polishing rate may tend to be low,or the planarity (flatness) of the polished object may tend to be low.If the cut rate is more than 2 μm/minute, the life of the polishing padmay be too short, which is not preferred.

The polyurethane foam preferably has a dimensional change rate of 0.8%or less when absorbing water, and preferably has a bending elasticmodulus change rate of 40% or less when absorbing water. If the value isoutside the range, the polishing layer may undergo a significantdimensional change when absorbing moisture or water, so that thepolishing characteristics such as planarization performance and in-planeuniformity may tend to be gradually degraded.

The polyurethane foam preferably has an Asker D hardness of 45 to 65degrees. If the Asker D hardness is less than 45 degrees, the planarityof the polished object may tend to be reduced. If the Asker D hardnessis more than 65 degrees, the in-plane uniformity of the polished objectmay tend to be reduced, although the polished object will have goodplanarity. In such a case, the surface of the object being polished canalso be easily scratched.

The polyurethane foam preferably contains 0.05 to 10% by weight of asilicone surfactant. If the content of the silicone surfactant is lessthan 0.05% by weight, it may tend to be difficult to produce amicrocellular foam. If the content of the silicone surfactant is morethan 10% by weight, it may tend to be difficult to obtain ahigh-hardness polyurethane foam due to the plasticizing effect of thesurfactant.

The invention is also related to a method for manufacturing a polishingpad, including the step of mixing a first component containing anisocyanate-terminated prepolymer with a second component containing achain extender and curing the mixture to form a polyurethane foam,wherein

the step includes adding a nonionic silicone surfactant to the firstcomponent containing the isocyanate-terminated prepolymer so that thepolyurethane foam will contain 0.05 to 10% by weight of the nonionicsilicone surfactant, stirring the first component together with anon-reactive gas to form a cell dispersion liquid in which thenon-reactive gas is dispersed in the form of fine cells, then mixing thesecond component containing the chain extender into the cell dispersionliquid, and curing the mixture to form the polyurethane foam,

the isocyanate-terminated prepolymer is

(1) an isocyanate-terminated prepolymer (A) includes an isocyanatemonomer, a high molecular weight polyol (a), and a low molecular weightpolyol, and(2) an isocyanate-terminated prepolymer (B) includes a polymerizeddiisocyanate and a polyethylene glycol with a number average molecularweight of 200 to 1,000.

The invention is also related to a method for manufacturing asemiconductor device, including the step of polishing a surface of asemiconductor wafer using the polishing pad.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a typical polishing apparatus foruse in CMP polishing; and

FIG. 2 is a schematic diagram showing 25 points on a wafer at each ofwhich the film thickness is measured.

DETAILED DESCRIPTION OF THE INVENTION

The polishing pad of the invention includes a polishing layer includinga polyurethane foam having fine cells. The polishing pad of theinvention may be only the polishing layer or a laminated body of thepolishing layer and any other layer (such as a cushion layer).

Polyurethane is a preferred material for forming the polishing layer,because polyurethane is excellent in abrasion resistance and polymerswith desired physical properties can be easily obtained by varying theraw material composition.

The polyurethane resin includes a cured product of a reaction of: anisocyanate-terminated prepolymer (A) includes an isocyanate monomer, ahigh molecular weight polyol (a), and a low molecular weight polyol; anisocyanate-terminated prepolymer (B) includes a polymerized diisocyanateand a polyethylene glycol with a number average molecular weight of 200to 1,000; and a chain extender.

As the isocyanate monomer, a compound known in the field of polyurethanecan be used without particular limitation. The isocyanate monomerincludes, for example, aromatic diisocyanates such as 2,4-toluenediisocyanate, 2,6-toluene diisocyanate, 2,2′-diphenyl methanediisocyanate, 2,4′-diphenyl methane diisocyanate, 4,4′-diphenyl methanediisocyanate, 1,5-naphthalene diisocyanate, p-phenylene diisocyanate,m-phenylene diisocyanate, p-xylylene diisocyanate and m-xylylenediisocyanate, aliphatic diisocyanates such as ethylene diisocyanate,2,2,4-trimethyl hexamethylene diisocyanate and 1,6-hexamethylenediisocyanate, and cycloaliphatic diisocyanates such as 1,4-cyclohexanediisocyanate, 4,4′-dicyclohexyl methane diisocyanate, isophoronediisocyanate and norbornane diisocyanate. These may be used alone or asa mixture of two or more thereof. Among the above isocyanate monomers,toluene diisocyanate and dicyclohexylmethane diisocyanate are preferablyused in combination.

As used herein, the term ‘polymerized diisocyanate’ refers to any ofpolymerized isocyanate derivatives produced by addition of three or moremolecules of diisocyanate, or refers to a mixture of the isocyanatederivatives. For example, the isocyanate derivative may be of (1)trimethylolpropane adduct type, (2) biuret type, (3) isocyanurate type,or the like. In particular, the isocyanurate type or the biuret type ispreferred.

In the invention, the polymerized diisocyanate is preferably producedusing aliphatic diisocyanate, specifically 1,6-hexamethylenediisocyanate. The polymerized diisocyanate may also be a modificationsuch as a urethane-modified, allophanate-modified, or biuret-modifiedpolymerized diisocyanate.

As the high molecular weight polyol (a), those usually used in the artof polyurethane can be exemplified. Examples thereof include polyetherpolyols represented by polytetramethylene ether glycol and polyethyleneglycol; polyester polyols represented by polybutylene adipate; polyesterpolycarbonate polyols exemplified by reaction products of polyesterglycol such as polycaprolactone polyol or polycaprolactone and alkylenecarbonate; polyester polycarbonate polyols obtained by reacting ethylenecarbonate with polyvalent alcohol and the reacting the resultantreaction mixture with an organic dicarboxylic acid; and polycarbonatepolyols obtained by ester exchange reaction between polyhydroxylcompound and aryl carbonate. These may be used singly or in combinationof two or more kinds.

The number average molecular weight of the high molecular weight polyol(a) is not particularly limited, however, from the viewpoint of moduluscharacteristic of obtainable polyurethane resin it is preferably in therange of 500 to 5000, more preferably in the range of 1000 to 2000. Whenthe number average molecular weight is less than 500, a polyurethaneresin obtained therefrom do not have sufficient modulus characteristic,and is likely to be a brittle polymer. And thus, a polishing pad formedof such polyurethane resin is too hard, and result in occurrence ofscratch on surface of an object to be polished. Also it is undesiredfrom the viewpoint of life time of polishing pad because ablation ismore likely to occur. On the other hand, number average molecular weightexceeding 5000 is not favorable because a polishing pad formed of apolyurethane resin obtainable therefrom is too soft to obtainsufficiently satisfactory planarity.

The polyethylene glycol used as a polyol component of theisocyanate-terminated prepolymer (B) needs to have a number averagemolecular weight of 200 to 1,000, and preferably 250 to 650. If thenumber average molecular weight is less than 200, the crosslinkingdistance may be so short that water may be less likely to be retained,which makes it impossible to form a highly water-absorbing polyurethanefoam. As a result, the polishing pad has low slurry-holding ability andcannot provide higher polishing rate. On the other hand, if the numberaverage molecular weight is more than 1,000, the crosslinking distancemay be so long that the water-absorbing properties may be too high,which may cause a significant dimensional change upon water absorption.

The low molecular weight polyol is an essential raw material for theisocyanate-terminated prepolymer (A). Examples of the low molecularweight polyol include ethylene glycol, 1,2-propylene glycol,1,3-propylene glycol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol,2,3-butanediol, 1,6-hexanediol, neopentyl glycol,1,4-cyclohexanedimethanol, 3-methyl-1,5-pentanediol, diethylene glycol,triethylene glycol, 1,4-bis(2-hydroxyethoxy)benzene, trimethylolpropane,glycerin, 1,2,6-hexanetriol, pentaerythritol, tetramethylolcyclohexane,methylglucoside, sorbitol, mannitol, dulcitol, sucrose,2,2,6,6-tetrakis(hydroxymethyl)cyclohexanol, diethanolamine,N-methyldiethanolamine, and triethanolamine. One or more of thesepolyols may be used alone or in any combination. In addition, the lowmolecular weight polyol may also be used as a raw material for theisocyanate-terminated prepolymer (B) if necessary.

A low molecular weight polyamine such as ethylenediamine,tolylenediamine, diphenylmethanediamine, or diethylenetriamine may alsobe used as a raw material for the isocyanate-terminated prepolymer (A)and (B) concomitantly. An alcoholamine such as monoethanolamine,2-(2-aminoethylamino)ethanol, or monopropanolamine may also be usedconcomitantly. These materials may be used alone or two or more of thesemay be used concomitantly.

The amount of the low molecular weight polyol, the low molecular weightpolyamine, or the like is, although not limited particularly, preferablyfrom 10 to 25% by mole, based on the amount of all the active hydrogengroup-containing compounds used as raw materials for theisocyanate-terminated prepolymer (A), while it may be appropriatelydetermined depending on the desired properties of the polishing pad(polishing layer) to be produced.

When the isocyanate-terminated prepolymer (B) is prepared, thepolymerized diisocyanate and the polyethylene glycol are preferablymixed in such a manner that the NCO index falls within the range of 3 to5, more preferably within the range of 3 to 4.

On the other hand, in the process of preparing the isocyanate-terminatedprepolymer (A), the NCO index is generally, but not limited to, fromabout 1.5 to about 2.5.

In a case where a polyurethane foam is produced by means of a prepolymermethod, a chain extender is used in curing of a prepolymer. A chainextender is an organic compound having at least two active hydrogengroups and examples of the active hydrogen group include: a hydroxylgroup, a primary or secondary amino group, a thiol group (SH) and thelike. Concrete examples of the chain extender include: polyamines suchas 4,4′-methylenebis(o-chloroaniline) (MOCA),2,6-dichloro-p-phenylenediamine, 4,4′-methylenebis(2,3-dichloroaniline),3,5-bis(methylthio)-2,4-toluenediamine,3,5-bis(methylthio)-2,6-toluenediamine, 3,5-diethyltoluene-2,4-diamine,3,5-diethyltoluene-2,6-diamine, trimethylene glycol-di-p-aminobenzoate,polytetramethylene oxide-di-p-aminobenzoate,4,4′-diamino-3,3′,5,5′-tetraethyldiphenylmethane,4,4′-diamino-3,3′-diisopropyl-5.5′-dimethyldiphenylmethane,4,4′-diamino-3,3′,5,5′-tetraisopropyldiphenylmethane,1,2-bis(2-aminophenylthio)ethane,4,4′-diamino-3,3′-diethyl-5.5′-dimethyldiphenylmethane,N,N′-di-sec-butyl-4,4′-diaminophenylmethane,3,3′-diethyl-4,4′-diaminodiphenylmethane, m-xylylenediamine,N,N′-di-sec-butyl-p-phenylenediamine, m-phenylenediamine andp-xylylenediamine; low-moleculer-weight polyol component; and alow-molecular-weight polyamine component. The chain extenders describedabove may be used either alone or in mixture of two kinds or more.

The mixing ratio of the isocyanate-terminated prepolymer (A), theisocyanate-terminated prepolymer (B), and the chain extender may bevaried depending on the molecular weight of each material and thedesired physical properties of the polishing pad. The amount of additionof the isocyanate-terminated prepolymer (B) is preferably from 5 to 30parts by weight, more preferably from 10 to 20 parts by weight, based on100 parts by weight of the isocyanate-terminated prepolymer (A).Further, in order to obtain a polishing pad having desired polishingproperties, the number of isocyanate groups in the prepolymers ispreferably from 0.8 to 1.2, more preferably from 0.99 to 1.15 per thenumber of active hydrogen groups (hydroxyl groups and/or amino groups)in the chain extender. If the number of isocyanate groups is outside therange, insufficient curing could occur so that the required specificgravity or hardness could not be achieved, which tends to decrease thepolishing properties.

The polyurethane foam is preferably produced by melting method in viewof cost, working environment and so on, while it may be produced byapplication of any known urethane foaming techniques such as meltingmethod and solution technique.

According to the invention, the polyurethane foam production isperformed using a prepolymer process. Polyurethane resin produced byprepolymer process has a preferably excellent physical properties.

Note that an isocyanate-terminated prepolymer (A) and (B) with amolecular weight of the order in the range of from 800 to 5000 ispreferable because of excellency in workability and physical properties.

The polyurethane foam is produced by mixing a first component containingthe isocyanate-terminated prepolymers (A) and (B) with a secondcomponent containing the chain extender and curing the mixture.

The polyurethane foam may be produced by a method in which hollow beadsare added, a mechanical foaming method, a chemical foaming method, orthe like. While any of the methods may be used in combination, amechanical foaming method with the aid of a silicone surfactant made ofa copolymer of polyalkylsiloxane and polyether is particularlypreferred. Preferred examples of such a silicone surfactant includeSH-192 and L-5340 (manufactured by Dow Corning Toray Silicone Co., Ltd).

Various additives may be mixed; such as a stabilizer including anantioxidant, a lubricant, a pigment, a filler, an antistatic agent andothers.

Description will be given of an example of a method of producing athermosetting polyurethane foam of a fine cell type constituting apolishing pad (a polishing layer) below. A method of manufacturing sucha polyurethane foam has the following steps:

1) Foaming Step of Preparing Cell Dispersion Liquid

The step includes adding a silicone surfactant to the first componentcontaining the isocyanate-terminated prepolymers (A) and (B) so that thepolyurethane foam will contain 0.05 to 10% by weight of the siliconesurfactant and stirring the mixture in the presence of a non-reactivegas to form a cell dispersion liquid in which the non-reactive gas isdispersed in the form of fine cells. In a case where the prepolymer issolid at an ordinary temperature, the prepolymer is preheated to aproper temperature and used in a molten state.

2) Curing Agent (Chain Extender) Mixing Step

The second component containing a chain extender is added into the celldispersion liquid, which is agitated to thereby obtain a foamingreaction liquid.

3) Casting Step

The forming reaction liquid is cast into a mold.

4) Curing Step

The foaming reaction liquid having been cast into the mold is heated andreaction-cured.

The non-reactive gas used for forming fine cells is preferably notcombustible, and is specifically nitrogen, oxygen, a carbon dioxide gas,a rare gas such as helium and argon, and a mixed gas thereof, and theair dried to remove water is most preferable in respect of cost.

As a stirrer for dispersing the silicone surfactant-containing firstcomponent to form fine cells with the non-reactive gas, known stirrerscan be used without particular limitation, and examples thereof includea homogenizer, a dissolver, a twin-screw planetary mixer etc. The shapeof a stirring blade of the stirrer is not particularly limited either,but a whipper-type stirring blade is preferably used to form fine cells.

In a preferable mode, different stirrers are used in stirring forforming a cell dispersion liquid in the stirring step and in stirringfor mixing an added chain extender in the mixing step, respectively. Inparticular, stirring in the mixing step may not be stirring for formingcells, and a stirrer not generating large cells is preferably used. Sucha stirrer is preferably a planetary mixer. The same stirrer may be usedin the stirring step and the mixing step, and stirring conditions suchas revolution rate of the stirring blade are preferably regulated asnecessary.

In the method of producing the polyurethane foam with fine cells,heating and post-curing of the foam obtained after casting and reactingthe forming reaction liquid in a mold until the dispersion lost fluidityare effective in improving the physical properties of the foam, and areextremely preferable. The forming reaction liquid may be cast in a moldand immediately post-cured in a heating oven, and even under suchconditions, heat is not immediately conducted to the reactivecomponents, and thus the diameters of cells are not increased. Thecuring reaction is conducted preferably at normal pressures to stabilizethe shape of cells.

In the production of the polyurethane foam, a known catalyst promotingpolyurethane reaction, such as tertiary amine-based catalysts, may beused. The type and amount of the catalyst added are determined inconsideration of flow time in casting in a predetermined mold after themixing step.

Production of the polyurethane foam may be in a batch system where eachcomponent is weighed out, introduced into a vessel and mixed or in acontinuous production system where each component and a non-reactive gasare continuously supplied to, and stirred in, a stirring apparatus andthe resulting forming reaction liquid is transferred to produce moldedarticles.

A manufacturing method of a polishing pad may be performed in ways: inone of which a prepolymer which is a raw material from which a polishingpad (a polishing layer) is made is put into a reaction vessel,thereafter a chain extender is mixed into the prepolymer, the mixture isagitated, thereafter the mixture is cast into a mold with apredetermined size to thereby prepare a block and the block is slicedwith a slicer like a planer or a band saw; and in another of which inthe step of casting into the mold, a thin sheet may be directlyproduced. Besides, a still another way may be adopted in which a resinof raw material is melted, the melt is extruded through a T die tothereby mold a polyurethane foam directly in the shape of a sheet.

The average cell diameter of the polyurethane foam is preferably from 20to 70 μm, more preferably from 30 to 60 μm. The polyurethane foampreferably has a cut rate of 2 μm/minute or less, and more preferably1.5 μm or less. The polyurethane foam preferably has a dimensionalchange rate of 0.8% or less, and more preferably 0.7% or less, whenabsorbing water. The polyurethane foam preferably has a bending elasticmodulus change rate of 40% or less, and more preferably 35% or less,when absorbing water. These physical properties may be measured by themethods described in the section “Examples”.

A hardness of a polyurethane foam is preferably in the range of from 45to 65 degrees, more preferably in the range of from 50 to 60 degrees asmeasured with an Asker D hardness meter.

A polishing pad (polishing layer) of the invention is provided with adepression and a protrusion structure for holding and renewing a slurry.Though in a case where the polishing layer is formed with a fine foam,many openings are on a polishing surface thereof which works so as tohold the slurry, a depression and protrusion structure are preferablyprovided on the surface of the polishing side thereof in order toachieve more of holdability and renewal of the slurry or in order toprevent induction of dechuck error, breakage of a wafer or decrease inpolishing efficiency. The shape of the depression and protrusionstructure is not particularly limited insofar as slurry can be retainedand renewed, and examples include latticed grooves, concentriccircle-shaped grooves, through-holes, non-through-holes, polygonalprism, cylinder, spiral grooves, eccentric grooves, radial grooves, anda combination of these grooves. The groove pitch, groove width, groovethickness etc. are not particularly limited either, and are suitablydetermined to form grooves. These depression and protrusion structureare generally those having regularity, but the groove pitch, groovewidth, groove depth etc. can also be changed at each certain region tomake retention and renewal of slurry desirable.

The method of forming the depression and protrusion structure is notparticularly limited, and for example, formation by mechanical cuttingwith a jig such as a bite of predetermined size, formation by castingand curing resin in a mold having a specific surface shape, formation bypressing resin with a pressing plate having a specific surface shape,formation by photolithography, formation by a printing means, andformation by a laser light using a CO₂ gas laser or the like.

No specific limitation is placed on a thickness of a polishing layer,but a thickness thereof is about 0.8 to 4 mm, preferably 1.5 to 2.5 mm.The method of preparing the polishing layer of this thickness includes amethod wherein a block of the fine-cell foam is cut in predeterminedthickness by a slicer in a band saw system or a planing system, a methodthat involves casting resin into a mold having a cavity of predeterminedthickness and curing the resin, a method of using coating techniques andsheet molding techniques, etc.

The scatter of the thickness of the polishing layer is preferably 100 μmor less. When the scatter of the thickness is higher than 100 μm, largeundulation is caused to generate portions different in a contactingstate with an object of polishing, thus adversely influencing polishingcharacteristics. To solve the scatter of the thickness of the polishinglayer, the surface of the polishing layer is dressed generally in aninitial stage of polishing by a dresser having abrasive grains ofdiamond deposited or fused thereon, but the polishing layer outside ofthe range described above requires a longer dressing time to reduce theefficiency of production.

As a method of suppressing the scatter of thickness, there is also amethod of buffing the surface of the polishing layer having apredetermined thickness. Buffing is conducted preferably stepwise byusing polishing sheets different in grain size.

A polishing pad of the invention may also be a laminate of a polishinglayer and a cushion sheet adhered to each other.

The cushion sheet (cushion layer) compensates for characteristics of thepolishing layer. The cushion layer is required for satisfying bothplanarity and uniformity which are in a tradeoff relationship in CMP.Planarity refers to flatness of a pattern region upon polishing anobject of polishing having fine unevenness generated upon patternformation, and uniformity refers to the uniformity of the whole of anobject of polishing. Planarity is improved by the characteristics of thepolishing layer, while uniformity is improved by the characteristics ofthe cushion layer. The cushion layer used in the polishing pad of thepresent invention is preferably softer than the polishing layer.

The material forming the cushion layer is not particularly limited, andexamples of such material include a nonwoven fabric such as a polyesternonwoven fabric, a nylon nonwoven fabric or an acrylic nonwoven fabric,a nonwoven fabric impregnated with resin such as a polyester nonwovenfabric impregnated with polyurethane, polymer resin foam such aspolyurethane foam and polyethylene foam, rubber resin such as butadienerubber and isoprene rubber, and photosensitive resin.

Means for adhering the polishing layer to the cushion layer include: forexample, a method in which a double-sided tape is sandwiched between thepolishing layer and the cushion layer, followed by pressing.

The double-sided tape is of a common construction in which adhesivelayers are provided on both surfaces of a substrate such as a nonwovenfabric or a film. It is preferable to use a film as a substrate withconsideration given to prevention of permeation of a slurry into acushion sheet. A composition of an adhesive layer is, for example, of arubber-based adhesive, an acrylic-based adhesive or the like. Anacrylic-based adhesive is preferable because of less of a content ofmetal ions, to which consideration is given. Since a polishing layer anda cushion sheet is sometimes different in composition from each other,different compositions are adopted in respective adhesive layers ofdouble-sided tape to thereby also enable adhesive forces of therespective adhesive layers to be adjusted to proper values.

A polishing pad of the invention may be provided with a double-sidedtape on the surface of the pad adhered to a platen. As the double-sidedtape, a tape of a common construction can be used in which adhesivelayers are, as described above, provided on both surfaces of asubstrate. As the substrate, for example, a nonwoven fabric or a film isused. Preferably used is a film as a substrate since separation from theplaten is necessary after the use of a polishing pad. As a compositionof an adhesive layer, for example, a rubber-based adhesive or anacrylic-based adhesive is exemplified. Preferable is an acrylic-basedadhesive because of less of metal ions in content to which considerationis given.

A semiconductor device is fabricated after operation in a step ofpolishing a surface of a semiconductor wafer with a polishing pad. Theterm, a semiconductor wafer, generally means a silicon wafer on which awiring metal and an oxide layer are stacked. No specific limitation isimposed on a polishing method of a semiconductor wafer or a polishingapparatus, and polishing is performed with a polishing apparatusequipped, as shown in FIG. 1, with a polishing platen 2 supporting apolishing pad (a polishing layer) 1, a polishing head 5 holding asemiconductor wafer 4, a backing material for applying a uniformpressure against the wafer and a supply mechanism of a polishing agent3. The polishing pad 1 is mounted on the polishing platen 2 by adheringthe pad to the platen with a double-sided tape. The polishing platen 2and the polishing head 5 are disposed so that the polishing pad 1 andthe semiconductor wafer 4 supported or held by them oppositely face eachother and provided with respective rotary shafts 6 and 7. A pressuremechanism for pressing the semiconductor wafer 4 to the polishing pad 1is installed on the polishing head 5 side. During polishing, thesemiconductor wafer 4 is polished by being pressed against the polishingpad 1 while the polishing platen 2 and the polishing head 5 are rotatedand a slurry is fed. No specific limitation is placed on a flow rate ofthe slurry, a polishing load, a polishing platen rotation number and awafer rotation number, which are properly adjusted.

Protrusions on the surface of the semiconductor wafer 4 are therebyremoved and polished flatly. Thereafter, a semiconductor device isproduced therefrom through dicing, bonding, packaging etc. Thesemiconductor device is used in an arithmetic processor, a memory etc.

Examples

Description will be given of the invention with examples, while theinvention is not limited to description in the examples.

[Measurement and Evaluation Method] (Measurement of Number-AverageMolecular Weight)

A number-average molecular weight was measured by GPC (a Gel PermeationChromatography) and a value as measured was converted in terms ofstandard polystylene molecular weight, and the apparatus and conditionsin operation were as follows:

GPC apparatus was an apparatus manufactured by Shimadzu Corp., withModel Number of LC-10A.

Columns that were used in measurement were ones manufactured by PolymerLaboratories Co., in which three columns were in connection including(PL gel, 5 μm and 500 Å), (PL gel, 5 μm and 100 Å) and (PL gel, 5 μm and50 Å).

A flow rate was 1.0 ml/min.

A concentration was 1.0 g/l.

An injection quantity was 40 μl.

A column temperature was 40° C.

An eluent was tetrahydrofuran.

(Measurement of Average Cell Diameter)

The prepared polyurethane foam was sliced with a microtome cutter intomeasurement samples each with the thinnest possible thickness of 1 mm orless. A surface of a sample was photographed with a scanning electronmicroscope (S-3500N, Hitachi Science Systems Co., Ltd.) at amagnification of ×100. An effective circular diameter of each of allcells in an arbitrary area was measured with an image analyzing soft(manufactured by MITANI Corp. with a trade name WIN-ROOF) and an averagecell diameter was calculated from the measured values.

(Measurement of Specific Gravity)

Determined according to JIS Z8807-1976. A manufactured polyurethane foamcut out in the form of a strip of 4 cm×8.5 cm (thickness: arbitrary) wasused as a sample for measurement of specific gravity and left for 16hours in an environment of a temperature of 23±2° C. and a humidity of50%±5%. Measurement was conducted by using a specific gravity hydrometer(manufactured by Sartorius Co., Ltd).

(Measurement of Hardness)

Measurement is conducted according to JIS K6253-1997. A manufacturedpolyurethane foam cut out in a size of 2 cm×2 cm (thickness: arbitrary)was used as a sample for measurement of hardness and left for 16 hoursin an environment of a temperature of 23±2° C. and a humidity of 50%±5%.At the time of measurement, samples were stuck on one another to athickness of 6 mm or more. A hardness meter (Asker D hardness meter,manufactured by Kobunshi Keiki Co., Ltd.) was used to measure hardness.

(Measurement of Cut Rate)

The prepared polyurethane foam sheet (380 mmφ, 1.25 mm thick) wasattached to the platen of a polishing apparatus (MAT-BC15, manufacturedby MAT Inc.). The surface of the polyurethane foam sheet was dressedusing a dresser (spot type, manufactured by Mitsubishi MaterialsCorporation) under the conditions of a forced drive rotation speed of115 rpm, a platen rotation speed of 70 rpm, a dressing load of 7 pounds,a water absorption rate of 200 ml/minute, and a dressing time of 1.5hours. After the dressing, a 10 mm wide, 380 mm long strip sample wascut from the polyurethane foam sheet. The thickness of the sample wasmeasured at points spaced at intervals of 20 mm along the central part(9 points on one side, 18 points in total). The difference (wear amount)between the measured thicknesses of the central part and the each pointalong the central part was calculated, and then the average of thedifferences was calculated. The cut rate is calculated from thefollowing equation:

the cut rate (μm/minute)=(the average wear amount)/(1.5×60).

(Measurement of Dimensional Change Rate Upon Absorption of Water)

The measurement was performed according to JIS K 7312. The resultingpolyurethane foam was cut into a sample 20 mm in width, 50 mm in length,and 1.27 mm in thickness. The sample was immersed in distilled water at25° C. for 48 hours, and the dimensional change rate was calculated bysubstituting its lengths before and after the immersion into thefollowing formula: dimensional change rate (%)=[(the length after theimmersion−the length before the immersion)/(the length before theimmersion)]×100.

(Measurement of Water Absorption Rate)

The resulting polyurethane foam was cut into a sample 20 mm in width, 50mm in length, and 1.27 mm in thickness. The sample was immersed indistilled water at 25° C. for 48 hours, and the water absorption ratewas calculated by substituting its weights before and after theimmersion into the following formula: water absorption rate (%)=[(theweight after the immersion−the weight before the immersion)/(the weightbefore the immersion)]×100. The water absorption rate is preferably 4.5%or more.

(Measurement of Bending Elastic Modulus Change Rate Upon WaterAbsorption)

A sample of a size of 1 mm width×3 mm length×2 mm thickness was cut fromthe prepared polyurethane foam. The bending elastic modulus of thesample was measured using a measuring system (Desktop Testing System5864, manufactured by Instron Corporation) under the followingconditions:

bending strength measuring tool, 22 mm in distance between fulcrums;crosshead speed, 0.6 mm/minute;amount of displacement, 6.0 mm.

The sample was also immersed in distilled water at 25° C. for 48 hours.The bending elastic modulus of the sample was then measured by the samemethod as described above. The bending elastic modulus change rate uponwater absorption was calculated from the following equation:

the change rate (%)=[{(the bending elastic modulus in the drystate)−(the bending elastic modulus after the immersion)}/(the bendingelastic modulus in the dry state)]×100

(Evaluation of Polishing Characteristics)

The prepared polishing pad was used to evaluate polishingcharacteristics by using a polishing apparatus SPP600S (manufactured byOkamoto Machine Tool Works, Ltd.). An about 1 μm thermal-oxide filmdeposited on an 8-inch silicone wafer was polished by 0.5 μm per oneplate, and polishing rate was calculated from the time of thispolishing. Table 1 shows the polishing rate of the 100th, 300th and500th wafers. The thickness of the oxide film was measured by using aninterference film thickness measuring instrument (manufactured by OtsukaElectronics Co., Ltd). During polishing, silica slurry (SS12manufactured by Cabot Microelectronics Co., Ltd) was added at a flowrate of 150 ml/min. Polishing loading was 350 g/cm², the number ofrevolutions of the polishing platen was 35 rpm, and the number ofrevolutions of the wafer was 30 rpm.

For evaluation of planarizing characteristics, a 0.5 μm thermal-oxidefilm was deposited on an 8-inch silicone wafer and subjected topredetermined patterning, and then a 1 μm oxide film of p-TEOS wasdeposited thereon, to prepare a wafer having a pattern with an initialdifference in level of 0.5 μm. This wafer was polished under theabove-described conditions.

For planarizing characteristics, an abrasion loss was measured. In twopatterns, that is, a pattern having lines of 270 μm in width and spacesof 30 μm arranged alternately and a pattern having lines of 30 μm inwidth and spaces of 270 μm arranged alternately, the abrasion loss of270 μm spaces was measured when the difference in level of the top ofthe line in the two patterns became 2000 Å or less. A lower abrasion ofspaces is indicative of higher planarity with less abrasion of portionsdesired to be not shaved. Table 1 shows the abrasion loss of the 100th,300th and 500th wafers.

The in-plane uniformity was evaluated as described below. An 8-inchsilicon wafer having a 1 μm-thick thermal-oxide film deposited thereonwas polished under the conditions described above for 2 minutes. Asshown in FIG. 2, the thickness was measured at specific 25 points on thewafer before and after the polishing. The maximum polishing rate and theminimum polishing rate were determined from the thickness of the filmmeasured at specific 25 points on the wafer before and after thepolishing, and the in-plane uniformity was calculated by substitutingthe resulting values into the formula below. Table 1 shows the in-planeuniformity of the 100th, 300th and 500th wafers. Note that a reducedvalue of in-plane uniformity means a higher uniformity on a wafersurface. In-plane uniformity (%)={(the maximum polishing rate−theminimum polishing rate)/(the maximum polishing rate+the minimumpolishing rate)}×100

Example 1

To a vessel were added 1,229 parts by weight of toluene diisocyanate (amixture of toluene 2,4-diisocyanate/toluene 2,6-diisocyanate=80/20), 272parts by weight of 4,4′-dicyclohexylmethane diisocyanate, 1,901 parts byweight of polytetramethylene ether glycol with a number averagemolecular weight of 1,018, and 198 parts by weight of diethylene glycol.The mixture was allowed to react at 70° C. for 4 hours to give anisocyanate-terminated prepolymer (A).

To a vessel were added 100 parts by weight of polymerized1,6-hexamethylene diisocyanate (Sumijule N-3300 (isocyanurate type)manufactured by Sumika Bayer Urethane Co., Ltd.) and 19.5 parts byweight of polyethylene glycol with a number average molecular weight of300 (NCO index: 4). The mixture was allowed to react at 100° C. for 3hours to give an isocyanate-terminated prepolymer (B1).

To a polymerization vessel were added 100 parts by weight of theprepolymer (A), 12 parts by weight of the prepolymer (B1), and 3.4 partsby weight of a silicone surfactant (SH-192 manufactured by Dow CorningToray Silicone Co., Ltd.), and mixed. The mixture was adjusted to 70° C.and reduced in pressure and degassed. The mixture was then vigorouslystirred with a stirring blade at a rotation number of 900 rpm for about4 minutes in such a manner that bubbles were incorporated into thereaction system. To the mixture was added 31.3 parts by weight of4,4′-methylenebis(o-chloroaniline) which had been previously melted at120° C. The mixture liquid was stirred for about 70 seconds and thenpoured into a loaf-shaped open mold (casting vessel). When the mixtureliquid lost its fluidity, it was placed in an oven and subjected to postcuring at 100° C. for 16 hours so that a polyurethane foam block wasobtained.

While heated at about 80° C., the polyurethane foam block was slicedusing a slicer (VGW-125 manufactured by AMITEC Corporation) so that apolyurethane foam sheet was obtained. The surface of the sheet was thenbuffed with a buffing machine (manufactured by AMITEC Corporation) untilthe sheet had a thickness of 1.27 mm. As a result, the sheet hadadjusted thickness accuracy. The buffed sheet was formed by punching tohave a diameter of 61 cm, and concentric grooves, 0.25 mm in width, 1.50mm in pitch, and 0.40 mm in depth, were formed in the surface of thesheet using a grooving machine (manufactured by Techno Corporation) sothat a polishing sheet (polishing layer) was obtained. A double-sidedtape (Double Tack Tape manufactured by Sekisui Chemical Co., Ltd.) wasbonded to the surface of the polishing sheet opposite to the groovedsurface using a laminator. The surface of a corona-treated cushion sheet(Toraypef manufactured by Toray Industries, Inc. (0.8 μm-thickpolyethylene foam)) was buffed. The buffed cushion sheet was bonded tothe double-sided tape using a laminator. Another double-sided tape wasalso bonded to the other side of the cushion sheet using a laminator sothat a polishing pad was prepared.

Example 2

To a vessel were added 100 parts by weight of polymerized1,6-hexamethylene diisocyanate (Sumijule N-3300 (isocyanurate type)manufactured by Sumika Bayer Urethane Co., Ltd.) and 38.9 parts byweight of polyethylene glycol with a number average molecular weight of600 (NCO index: 4). The mixture was allowed to react at 100° C. for 3hours to give an isocyanate-terminated prepolymer (B2).

A polishing pad was prepared using the process of Example 1, except that14 parts by weight of the prepolymer (B2) was used in place of 12 partsby weight of the prepolymer (B1).

Example 3

To a vessel were added 100 parts by weight of polymerized1,6-hexamethylene diisocyanate (Sumijule N-3300 (isocyanurate type)manufactured by Sumika Bayer Urethane Co., Ltd.) and 64.8 parts byweight of polyethylene glycol with a number average molecular weight of1000 (NCO index: 4). The mixture was allowed to react at 100° C. for 3hours to give an isocyanate-terminated prepolymer (B3). A polishing padwas prepared using the process of Example 1, except that 16.5 parts byweight of the prepolymer (B3) was used in place of 12 parts by weight ofthe prepolymer (B1), the amount of addition of the silicone surfactantwas changed from 3.4 parts by weight to 3.5 parts by weight, and theamount of addition of 4,4′-methylenebis(o-chloroaniline) was changedfrom 31.3 parts by weight to 30.8 parts by weight.

Comparative Example 1

A polishing pad was prepared using the process of Example 1, except thatthe prepolymer (B1) was not added, the amount of addition of SH-192 waschanged from 3.4 parts by weight to 3.0 parts by weight, and the amountof addition of 4,4′-methylenebis(o-chloroaniline) was changed from 31.3parts by weight to 26.6 parts by weight.

TABLE 1 Dimensional Water change absorption Bending elastic Average cellsize Specific D hardness Cut rate rate rate modulus change rate (μm)gravity (degrees) (μm/min) (%) (%) (%) Example 1 55 0.86 53 1.5 0.486.00 30 Example 2 52 0.86 51 1.3 0.55 6.80 33 Example 3 54 0.86 50 1.10.68 6.70 38 Comparative 53 0.86 50 1.0 0.90 3.00 55 Example 1 Polishingrate In-plane uniformity (Å/min) Abrasion loss (Å) (%) 100 300 500 100300 500 100 300 500 pieces pieces pieces pieces pieces pieces piecespieces pieces Example 1 2750 2750 2750 2800 2800 2850 5.6 6.8 8.0Example 2 2800 2800 2800 2900 2900 2900 5.5 7.7 9.2 Example 3 2800 28002800 2900 2950 2950 5.5 8.3 10.3 Comparative 2200 2200 2200 3000 32003400 7.5 10.0 20.0 Example 1

The results in Table 1 show that the polishing pad of the invention hasnot only high water-absorbing properties but also high dimensionalstability upon moisture absorption or water absorption and achieves ahigh level of polishing rate, planarization performance and in-planeuniformity and is less likely to exhibit variations in these properties.It is also apparent that the polishing pad of the invention particularlyprovides a high polishing rate.

1. A polishing pad, comprising a polishing layer comprising apolyurethane foam having fine cells, wherein the polyurethane foam is acured product of a reaction of (1) an isocyanate-terminated prepolymer(A) that includes an isocyanate monomer, a high molecular weight polyol(a), and a low molecular weight polyol, (2) an isocyanate-terminatedprepolymer (B) that includes a polymerized diisocyanate and apolyethylene glycol with a number average molecular weight of 200 to1,000, and (3) a chain extender.
 2. The polishing pad according to claim1, wherein the high molecular weight polyol (a) is a polyether polyolwith a number average molecular weight of 500 to 5,000, and theisocyanate monomer is toluene diisocyanate or dicyclohexylmethanediisocyanate.
 3. The polishing pad according to claim 1, wherein thepolymerized diisocyanate is a polymerized hexamethylene diisocyanate ofisocyanurate type and/or biuret type.
 4. The polishing pad according toclaim 1, wherein the isocyanate-terminated prepolymer (B) is added in anamount of 5 to 30 parts by weight, based on 100 parts by weight of theisocyanate-terminated prepolymer (A).
 5. The polishing pad according toclaim 1, wherein the polyurethane foam has an average cell diameter of20 μm to 70 μm and a cut rate of 2 μm/minute or less.
 6. The polishingpad according to claim 1, wherein the polyurethane foam has adimensional change rate of 0.8% or less and a bending elastic moduluschange rate of 40% or less when absorbing water.
 7. The polishing padaccording to claim 1, wherein the polyurethane foam contains 0.05 to 10%by weight of a silicone surfactant.
 8. A method for manufacturing apolishing pad, comprising the mixing a first component containingisocyanate-terminated prepolymer with a second component containing achain extender and curing the mixture to form a polyurethane foam,further comprising adding a nonionic silicone surfactant to the firstcomponent containing the isocyanate-terminated prepolymer so that thepolyurethane foam will contain 0.05 to 10% by weight of the nonionicsilicone surfactant, further stirring the first component together witha non-reactive gas to form a cell dispersion liquid in which thenon-reactive gas is dispersed in the form of fine cells, then mixing thesecond component containing the chain extender into the cell dispersionliquid, and curing the mixture to form the polyurethane foam, andwherein the isocyanate-terminated prepolymer is (1) anisocyanate-terminated prepolymer (A) that includes an isocyanatemonomer, a high molecular weight polyol (a), and a low molecular weightpolyol, and (2) an isocyanate-terminated prepolymer (B) that includes apolymerized diisocyanate and a polyethylene glycol with a number averagemolecular weight of 200 to 1,000.
 9. A method for manufacturing asemiconductor device, comprising of polishing a surface of asemiconductor wafer using the polishing pad according to claim 1.